1. Field of the Invention
The present invention relates to a carrier for use in two-component developers comprising a toner and a carrier, and more specifically relates to a binder-type carrier comprising magnetic powder dispersed in a binder resin and method of manufacturing the carrier.
2. Description of the Related Art
Image forming apparatuses, such as copiers and printers of the electrophotographic-type, which use a two-component developer including a toner and a carrier to develop an electrostatic latent image formed on an image-carrying member such as a photosensitive member or the like are known.
In recent years, however, organic photosensitive members provided with an organic photosensitive layer superimposed on sequential laminations of a charge-generating layer and a charge-transporting layer on an electrically conductive substrate have been proposed. These photosensitive members are said to have excellent photosensitivity, excellent stability and low manufacturing cost.
Such organic photosensitive members include a negative charging photosensitive member and a highly efficient normal hole transporting material as a charge-transporting material. Developing must be accomplished by a reverse developing method using a developer with a negatively chargeable toner in order to use the organic photosensitive member in a digital-type image forming apparatus. Therefore, a negatively chargeable two-component developer having excellent characteristics is required.
There are various known carriers including magnetic carriers, iron powder carrier, ferrite carrier, carriers covered by a resin containing magnetic powder or iron or ferrite, binder-type carriers comprising a magnetic powder dispersed in a binder resin and the like. Among these carriers, the binder-type carriers have gained attention as carriers which can be readily produced in small particle size, have a high volume specific resistivity, and resist charge injection from the developer-carrying member.
A carrier having a suitable chargeability relative to a negatively chargeable toner must have an amount of magnetic powder on the carrier surface within a suitable range to act as charging points for the negatively chargeable toner. The amount of magnetic powder on the carrier surface can be measured by dissociating the magnetic powder present on the carrier surface. This dissociation may be accomplished by introducing the carrier into a solvent, such as an acid or the like, capable of dissolving the magnetic powder.
However, adequate chargeability relative to negatively chargeable toner may not be obtained even if a suitable amount of surface magnetic powder is confirmed using this measurement method. The causes of this inadequate chargeability is thought to be due to uneven dispersion of the magnetic powder in the binder resin, wherein free magnetic powder is mixed in during the manufacturing process so as to produce flocculation of magnetic powder contained in the carrier. This disadvantage becomes more pronounced when the magnetic powder content is increased to improve the chargeability of the carrier relative to the negatively chargeable toner.
It is difficult to simply eliminate free magnetic powder by classification since, due to the large particle size difference of the carrier particles, it readily adheres to the carrier particles. Nonetheless, the disadvantage caused by this fine powder content can be eliminated by removing the free magnetic powder. The free magnetic powder can be eliminated by improving the precision of the classification process or increasing the number of classifications. In this case, however, the classification process becomes quite complex and reduces manufacturing efficiency.
Moreover, a further disadvantage of uneven image density occurs when this type carrier is used for image formation under conditions of high temperature and high humidity (H/H).